It just comes down to practice.
Alan Munde has a DVD covering finger stretching exercises.
THE BLUEGRASS BANJO WORKOUT DVD
In ten information-rich lessons, master teacher Alan Munde de-mystifies many right and left hand playing techniques. Work with these lessons will add to the player’s dexterity, will increase left hand stretch, solve difficult left hand fingerings, increase speed, and help with the visual organization of the fingerboard.
(93-MU-B2) $29.95
THE BLUEGRASS BANJO WORKOUT DVD
$29.95 plus shipping
Click on the link below and scroll to the bottom of the page
https://almundesbanjocollege.com/dvds/